Reusing sprite images
---------------------

While the gameduino can display 512 sprites per screen, it only has room
for 64 16x16 sprite images.

But by using a couple of techniques with the palette, you can stretch the
total to 128 or 256 sprite images.  The price you pay for the increase
is reduced color choice.

Normally, a each pixel in the sprite is a byte, and the gameduino uses this byte index
to look up a color in the sprite's 256-entry color palette.  The palette
contains 256 different colors, so any sprite pixel can be any of these
256 colors.

But if you choose a smaller palette of 16 colors, you can 

However, you can mix and match these approaches.  For example you might
use 256-color images for the 'hero' sprite images, 16-color images for
the assorted enemies, and 4-color images for text and icons.
